Use is conceivable wherever users of personal transmitter units must send under varying environmental conditions. Examples are found above all in case of the use of transmitter units arranged near the body when the user of these transmitter units is moving and reliable radio transmission is necessary.
Decisive advantages of wireless systems for data transmission compared to wired systems are applicability in a short time and low effort for installation, area-covering availability and, frequently, advantages in terms of handling. In case of installation near the body, in particular, there are hardly any limitations of the freedom of movement.
A further increase in the use of wireless radio transmission systems can be expected in the course of technical improvements. Hazard potentials or harmful effects that may occur or are linked with such a development are to be identified and eliminated with certainty.
The increasing saturation of the human environment with alternating electromagnetic fields is a circumstance that is invariably brought into connection with the possibility of sporadic harmful effects. In particular, digital signals rich in harmonics and high-frequency fields are sometimes at least assumed to have harmful effects on health or well-being, These harmful effects, which are taken into account, can be partially explained physically especially in connection with polar liquids; they are usually not detectable at relevant field intensities, but also cannot be read without certainty. However, the responsible management of technical infrastructure also requires that hypothetical harmful effects on health be also ruled out as much as possible.
The hazard potential is to be kept as low as possible in justified cases, in which such a hypothetical harmful effect must be accepted. Moreover, it is also useful to minimize the radiation exposure caused by alternating electromagnetic fields in order to further improve the acceptance of wireless systems, which could open up the utilization of undoubtedly existing advantages of such systems in additional fields of application.
Such a field of application is the use of physical sensors directly at the human body. Wired connection has decisive drawbacks here in terms of handling compared to the wireless connection. In case of the application of various sensors, in particular, the handling of these sensors should, however, be as simple as possible, and the user's free space of movement should be restricted as little as possible.
An especially sensitive subject is the generation of high-frequency electromagnetic fields directly near the body, which is certainly due at least partly to the wide popularization of high-frequency mobile telephones. The area near the body is to be defined in this connection as the area that is characterized by distances from the body surface that are similar to or shorter than the dimensions of the human body.
On the one hand, the use of transmitters for radio transmission requires certain minimum levels in order to guarantee reception; on the other hand, precisely the acceptance of arrangement near the body does offer decisive advantages concerning comfort, availability and possibilities of application of wireless transmission systems. Signals to be transmitted are frequently generated near the body. If they are transmitted at a location close to the site at which they are generated, this means a reduction of the effort, because the installation of means for forwarding the signals from the site of generation to a transmitter arranged separately can be extensively eliminated.
However, transmitters arranged directly near the body raise other technical problems. Part of the transmitting power may be absorbed by the tissues of the human body. This happens especially at high frequencies that are to be taken into account. This results in unfavorable radiation characteristics. If the human body is located between the transmitter and the reception antenna, this must be overcome by an increased transmitting power as an absorbing obstacle. The attenuation by water molecules is particularly effective especially at frequencies above 1.3 GHz. This is associated with an unintended strong signal attenuation. In addition, an undesired heating of the body fluids may also occur depending on the transmitting power, because the water molecules are excited to perform vibrations owing to their polar character. Such a high radiation exposure is especially problematic in the area of the head, which is sometimes brought into connection with the possibility of the development of damage to the eyes. Concentration of the transmitting power to the area of the head is therefore unfavorable. However, corresponding to the state of the art, especially in mobile radio technology, an especially intense release of transmitting power takes place precisely in this area.
It is possible, in principle, to overcome obstacles that may possibly be in the way by increasing the transmitting power in case of poor quality of transmission or poor reliability of reception.
It is, furthermore, known that the reception geometry can be optimized by the use of a plurality of reception antennas and possibly switching over between these antennas. The use of a plurality of receiving antennas in passenger cars shall be mentioned here as an example.